In some applications, optical surgical probes may deliver light to multiple spots at a surgical target. For example, in photocoagulation of retinal tissue, multiple spots may reduce the time of the procedure. Various techniques have been employed to produce multiple beams for a multi-spot pattern. For example, one approach uses diffractive elements at the distal end of the probe to divide an incoming beam into multiple beams.
Difficulties, however, can arise with using diffractive elements at the distal end of the probe. As one example, diffractive elements produce a multitude of higher diffraction orders. While these orders have a lower light intensity as compared to the primary spot pattern, they may still have some effect. As another example, a diffractive element may not perform the same in different refractive media. For example, a diffractive element may be placed into a medium with a different refractive index than that of air, and spaces between the diffractive elements may fill with the medium, which may affect the spot pattern. As yet another example, the spacing between the spots can vary for different wavelengths, which can cause problems if an aiming beam and a treatment beam are different colors. Lastly, diffractive elements are frequently expensive and difficult to produce, especially if the diffractive element is to fit into a small area, such as a distal tip of a surgical probe for surgical instruments that are 23-gauge or smaller.